Optical information recording media have attracted attention due to their high density and large capacity and have been used in various application uses. For instance, optical discs used exclusively for reading include, for example, compact discs or CD-ROM used exclusively for data reading, and they are used generally, for example, in the field of musics, computers and games. Further, writing once type optical discs capable of recording only once have been used in document filing systems and data filing systems, particularly, in the field in which data security is important.
Further, re-writable optical discs capable of erasing recorded information and re-recording are expected in that they contribute to extension for the application uses of the optical discs since they can correct or update data and can be used repeatedly by rewriting. For the re-writable optical discs described above, magneto-optic discs or phase-change type optical discs have been put to practical use and used in data files or the like.
In order to further increase the capacity of the optical disc system, it is necessary to increase the recording density. Then, for increasing the density, an effective method is to decrease a spot diameter of a laser. The spot diameter is in proportion with a laser wavelength and in inverse proportion with numerical aperture (NA) of an objective lens. Therefore, increase of NA is an effective means for decreasing the spot diameter but, as the NA increases, aberration increases relative to the tilt of a disc and the effect of the tilting angle ("Tilt") of the disc is increased.
Since the aberration is in proportion with the thickness of the substrate and (NA).sup.3, where NA should be increased, the aberration can be reduced by reducing the thickness of the substrate.
For example, in a 1.2 mm substrate used in conventional CD, if NA is increased to 0.6 or greater, the tilting angle is allowed to only about 4 Mrad for the disc, and it is not practical considering the actual using circumstance or productivity. However, when a substrate of 0.6 mm thickness is used for instance, the tilting angle of the disc can be permitted to about 8 Mrad even if NA is 0.6, and this is within a sufficiently practical usable range (T. Sugaya, et al.: Jpn. Appl. Phys. 32 (1993), 5402., T. Ohta, et al.: Jpn. Appl. Phys. 32 (1993) 5214).
Meanwhile, in the case of using the 0.6 mm substrate, since the recording capacity per volume can be doubled in simple way compared with the 1.2 mm substrate, decreasing thickness of the substrate is desirable also in order that the capacity can be increased without enlarging the size of a medium or a device.
As described above, a method of decreasing the thickness of the substrate is effective for reducing the negative effect by "Tilt", and 0.6 mm thickness will become predominant as the thickness of high density optical disc substrates in next generation in the feature.
On the other hand, plastic molding products are usually used as the substrate for the optical discs but, since the strength of a plastic substrate has a correlation with a thickness of the substrate, the strength of the substrate is weakened as the thickness is reduced. Therefore, in a plastic substrate with the thickness as thin as 0.6 mm, strain (partial deformation) 2 may sometimes occurs to a substrate 11 due to stress or heat caused in a thin film upon deposition of the film as shown in FIG. 1.
The strain caused to the substrate results a problem that no satisfactory mechanical accuracy can be obtained. That is, since the strain in the substrate worsens particularly focus acceleration, laser tracking is no more possible and off focus occurs to cause an error. In a worst case, focusing is out of the control to cause a significant trouble.
The strain in the substrate is attributable to the stress occurring in the entire thin films (entire stress) laminated on the substrate and the entire stress is determined as a product of the internal stress for the entire thin films and the entire film thickness. Accordingly, as a method of suppressing the strain in the plastic substrate, there may be considered a method for reducing the thickness of thin films disposed on the substrate, or a method for reducing the internal stress of the thin films by depositing the thin films with a material resulting less stress or by changing the film-depositing conditions.
However, since the methods decrease the degree of freedom for the film design and restrict the film-depositing conditions, they can not be always compatible with satisfactory signal characteristics.
The present invention has been accomplished taking notice on the problems in the prior art and it is a subject thereof to provide an optical disc comprising a plastic substrate with a thickness as thin as 0.6 mm, not showing the restriction for the film constitution or film-depositing conditions and free from strain of the substrate.